The present invention relates to a master cylinder for supplying a brake fluid to a brake apparatus of a vehicle.
As a conventional master cylinder for supplying a brake fluid to a brake apparatus of a vehicle, there is known a master cylinder comprising a cylinder formed by a cylinder body and a sleeve, which are disposed so as to form a discharge passage and a supply passage for a brake fluid. A piston is slidably disposed in the cylinder, to thereby form a pressure chamber for supplying a fluid pressure to the discharge passage between the piston and the cylinder. A piston seal capable of tightly disconnecting the supply passage and the pressure chamber from each other is provided between the cylinder and the piston. In this master cylinder in which a cylinder is formed by a sleeve and a cylinder body, the number of parts increases and the size of the cylinder radially increases, thus increasing the size of the master cylinder. Therefore, it has been proposed to use a master cylinder such as that disclosed in U.S. Pat. No. 4,524,585, in which a piston is directly inserted into a cylinder body without using a sleeve.
In this master cylinder, a cylinder body includes a communication groove which opens into a circumferential groove and extends from the circumferential groove toward a closed end of the cylinder body, to thereby allow communication between a discharge passage for a brake fluid and the circumferential groove. When releasing air from the cylinder body, a piston seal provided in the circumferential groove is opened under differential pressure, and the brake fluid is flowed toward the discharge passage through a gap between the piston seal and a bottom surface of the circumferential groove, and the communication groove. In this master cylinder, the communication groove is located inward of the bottom surface of the circumferential groove, relative to a radial direction of the cylinder body. Therefore, when releasing air by applying suction, due to the action of a negative pressure in the cylinder body, an end portion near the outer circumferential surface of the piston seal adheres to a side wall surface rising from the bottom surface of the circumferential groove, thereby preventing the brake fluid from flowing into the communication groove through the gap between the outer circumferential surface of the piston seal and the bottom surface of the circumferential groove. Therefore, the supply of the brake fluid is restricted and an air releasing operation must be repeated several times.